Systems which include means for converting analog signals to digital form, then compression filtering and encoding the compressed signals for recording or for transmission to a remote location, together with playback or receiver means which include a decoder, a digital reconstruction filter, and means for converting the decoded and filtered digital signals back to analog form are well known as shown in U.S. Pat. No. 4,449,536, issued May 22, 1984, by the present inventor. Similar systems are shown in the following articles: Ruttimann et al, "IEEE Transactions on Biomedical Engineering," vol. BME-26, No. 11, Nov. 1979, pp. 613-623 and Ripley et al, "Pro. Comput. Cardiol." 1976, pp. 439-445.
Truncated Huffman encoding is used in the above-mentioned systems, and another truncated Huffman code is shown in U.S. Pat. No. 4,396,906, issued Aug. 2, 1983, by the present inventor. The code disclosed in U.S. Pat. No. 4,396,906 includes a single 1's bit in the code word at the least significant bit position. Other bit positions of the code words contain only 0's. Infrequently occurring compressed signals are labelled, and the labelled compressed signals are provided at the output from the coder.
A Huffman code has the property that two or more code words cannot be placed in sequence to generate another member of the code word set. This property makes it possible to find the beginning and end of each code word without the need for an end-of-word symbol. A buffer memory is included in the playback or receiver means upstream of the decoder which allows for operation of the decoder at a substantially constant word rate even though the code words supplied thereto are of variable word length. However, when bit errors occur, and the decoder is not synchronized, bits will be taken from the buffer memory at an instantaneous rate that is greater than the rate those same bits were produced during the encoding process. A plurality of valid code words are required for each resynchronization. If there are many bit errors, such as may occur during transmission between transmitting and receiving units, there is a high probability of underflow of the receiver buffer memory means.